Giant spin-to-charge conversion at an all-epitaxial single-crystal-oxide Rashba interface with a strongly correlated metal interlayer

The two-dimensional electron gas (2DEG) formed at interfaces between SrTiO3 (STO) and other oxide insulating layers is promising for use in efficient spin-charge conversion due to the large Rashba spin-orbit interaction (RSOI). However, these insulating layers on STO prevent the propagation of a spin current injected from an adjacent ferromagnetic layer. Moreover, the mechanism of the spin-current flow in these insulating layers is still unexplored. Here, using a strongly correlated polar-metal LaTiO3+δ (LTO) interlayer and the 2DEG formed at the LTO/STO interface in an all-epitaxial heterostructure, we demonstrate giant spin-to-charge current conversion efficiencies, up to ~190 nm, using spin-pumping ferromagnetic-resonance voltage measurements. This value is the highest among those reported for all materials, including spin Hall systems. Our results suggest that the strong on-site Coulomb repulsion in LTO and the giant RSOI of LTO/STO may be the key to efficient spin-charge conversion with suppressed spin-flip scattering. Our findings highlight the hidden inherent possibilities of oxide interfaces for spin-orbitronics applications.

As a reference, we also grew a single LTO layer with a thickness of 20 u.c. on STO. The out-of-plane X-ray diffraction (XRD) pattern and reciprocal space mapping for both LSMO/LTO/STO and LTO/STO samples show only pseudo-cubic (00l) peaks with Laue (Kiessig) fringes (see the dotted square in Supplementary Fig. 3a), confirming that these samples are single-phase with abrupt interfaces and have no discernible La2Ti2O7 phase ( Fig. 1e and Supplementary Fig. 3, 4).

Supplementary Note 3: Estimation of the relaxation time τe of electrons
The relation between the two-dimensional current jC 2D and the electric field F in the x direction is expressed by where n is the index of the Fermi surface FSn, e is the free electron charge, ℏ is the Dirac The obtained τe at each temperature T is shown in Supplementary Fig. 8. 2. From the band structure obtained in process 1, we theoretically calculated the Fermi level EF dependence of the DOS and nsheet. We measured the Hall effect in LTO (3 u.c.)/STO, from which we estimated nsheet to be ~8.9×10 13 cm -2 at 4 K. We determined the Fermi level E'F of the LSMO/LTO/STO sample from this nsheet using the calculated EF dependence of nsheet (see Supplementary Fig. 9).
3. We checked whether the theoretical λIEE using the E'F value obtained above can reproduce the experimental λIEE or not. If not, we went back to process 1, changed the values of ΔASO and Δz, and followed the process 1 to 3 again.
After repeating the above-mentioned procedure, we found that the conditions of ΔASO = 0.015 eV and Δz = 0.02 eV can well reproduce the experimental λIEE. In Supplementary   Fig. 9, one can see that EF = E'F = -0.102 eV corresponds to nsheet = ~8.9×10 13 cm -2 .
The differences in nsheet and EF of LTO/STO between the measurement methods, i.e.
R-ARPES and the Hall measurement, may be due to the difference in oxidization of the sample. For the R-ARPES measurement, the oxidation was suppressed because the sample was transferred under a full nitrogen atmosphere. Meanwhile, for the Hall measurement, the sample was exposed to the atmosphere, and thus the surface was more Fermi level is estimated to be located at ⁓0.3 eV higher than the 1st dxy subband bottom (see Fig. 4a in the main manuscript).
As we mentioned above, we measured the Hall effect for the LTO (3 u.c.)/STO sample, for which the sheet carrier density nsheet, mobility and the sheet resistance Rsheet were estimated to be ~8.9×10 13 cm -2 , 6.9×10 3 cm 2 /Vs and ~8 Ω/□ at 4 K, respectively. reported thus far S1−S12 . The dark orange area is the range of Rsheet obtained in our LSMO/LTO/STO and LTO/STO heterostructures.